The invention relates to motion pictures and more specifically to an analog sound track digitizer for motion pictures.
The best prior art analog sound track digitizer is described and illustrated in the Carlsen U.S. Pat. No. 5,526,075. This earlier version of the inventors scanner was found difficult to adjust for the variations in errors in film sound tracks. Some of these included variations in the location of the sound track or weave, the minimum width of the tracks when there was no modulation, changes in film density, scratches, blotches and the inability to deal with very large noise transients caused by breaks in the film and splices.
The present standard for sound-tracks imaged on motion picture film dates back to 1967 wherein the location on, and the area covered by stereo sound tracks on film was specified. This standard describes the dimensions of the track and the related standards used today. This method places two tracks along one edge of the film which are of two types, variable density and variable area, the last of which is used almost exclusively. Inherent to this method of reproducing sound is the undesirable effect of background noise and rumble due to the nature of the plastic medium and residue of the film emulsion passing through the light beam of the photo sound detector. Dolby Corporation with its Dolby B,C,H, etc has pretty much dominated the industry both with its noise reduction systems for movie film and those for magnetic tape. However even with the best system Dolby can offer, the sound suffers distortion and amplitude variations caused by the extreme processing needed to remove random noise from between the normal sound peaks.
The introduction of the compact disk or CD with its almost perfect sound reproduction ability has stimulated interest by several companies to try and incorporate digital-quality sound on film.
This has proven to be a formidable problem because of the immense amount of digital information required to produce the multiple channel digital formats while keeping the old stereo sound tracks intact for general use by most movie theaters. Also the cost of the digital reader-heads to decode the new digital sound tracks is very high. Dolby Laboratories has recently developed its Theater Digital System that is currently being tested in a number of theaters. Sony Corporation also has introduced their digital system. It should be noted that these systems are expensive and complicated. Also the economics of necessary maintenance and the actual working-life of a digitally-encoded film sound-track is yet to be established.
None of the new digital sound systems that are being developed by these major corporations at great expense, does anything to improve old analog film sound-track that remain on the film.
The background of the present invention relates to the reproduction of sound from motion picture film. Two of the biggest hurdles to overcome in the effort to improve sound from the movie film are the increase of high frequency response and the reduction of background noise. Filters to reduce background noise also reduce high frequency response which forces the user to use compression and dynamic filter techniques. These lead to unwanted distortions and complexity.
Present analog sound track readers used in moving picture theater projectors read the variable width sound tracks on motion picture film by back lighting the sound track portion of the film with a focused slit of light which is arranged perpendicular to 1l the direction of travel of the film and just wide enough to span both tracks of the normal stereo print. A dual photo detector is placed on the opposite sides of the film to intercept the light which passes through the sound track portion of the film. The two tracks, which are transparent to light, vary in width as the film moves past the slit light source. The thickness of the slit light source and the instantaneous width of the sound track as it passes the photo detector determines the total amount of light falling on the photo detector and in turn the amount of electrical output from the detector. As the film continues to move past the detector the sound information which modulates the width of the tracks is converted to an electrical audio signal. Present photodiode detectors are linear devices and any change in the amount of light falling on the detector causes a corresponding electrical output. Electrical output changes due to different changes in the width of the sound tracks cause useful output. However changes due to residual emulsion, scratches, dirt and light aberration through the film plastic medium cause unwanted light modulation and are perceived as background noises in the electrical output.
The frequency response of present systems, irrespective of the noise limitation, is ultimately limited by the recording camera response. In reality, the limitation is based on the thickness of the slit light source used during playback. The best of the slit lenses produce about a 0.5 mil slit thickness which produces a high frequency limit of about 5 KHZ. These wide band slit lenses are usually used in special playback systems such as Dolby stereo or Surround Sound and are followed by dynamic noise filters high frequency boost amplifiers and expanders. These require that the film be specially recorded with compression techniques to improve signal and noise ratios. Generally slit sizes are used which produce high frequency limits of about 9 to 16 KHZ.
It is an object of the invention to provide an improved analog sound track digitizer having the highest accuracy and having a minimum amount of noise and distortion.
It is also an object of the invention to provide an improved analog sound track digitizer having a novel weave tracking circuit.
It is another object of the invention to provide an improved analog sound track digitizer having a circuit which allows the scanner to track minimum width tracks that are as little as {fraction (1/4+L )}the minimum standard set by the industry. 
It is an additional object of the invention to provide an improved analog sound track digitizer having an automatic gain control (AGC) that was added to the video amplifier circuit which allows for changes of 10 to 1 in light density and/or film density.
It is a further object of the invention to provide an improved analog sound track digitizer having a new method for comparing the video signal with a reference signal.
It is another object of the invention to provide an improved analog sound track digitizer having novel noise canceling circuits which work independently to remove the effect of breaks or scratches as large as 1/10 of an inch on the film and blotches of any size.
The novel analog sound track digitizer for motion picture projectors has been designed to be retrofit to existing sound head housings but the system can also be incorporated in newly manufactured sound head housings.
The present invention includes structure for linearly scanning a light beam of appropriate dimensions and intensity perpendicularly across the movie film sound tracks. A photo detector is placed on the opposite side of the film to intercept the light beam such that when the light beam is directed on the dark or emulsion portion of the film there is no output from the photo detector. When the light beam is directed on the transparent portion of the film the photo detector is saturated. The resulting output of the photo detector is a group of electrical pulses each having a width proportional to the width of the related transparent portion of the film sound tracks and all having a fixed amplitude. Continuously scanning the light beam at a fixed rate and frequency across the films sound tracks produces a continuous stream of pulses each changing in width at a rate related to the instantaneous changes in the width of the transparent portions of the sound track. The scanning frequency of the beam is chosen to produce the desired high frequency response of the system, usually twice the desired frequency.
The resulting width modulated pulse streams are passed through level comparators then integrators which convert the pulse streams to audio signals. The benefits of this method are first that since the detected pulses are either zero amplitude at dark emulsion or saturated at transparent, all noise due to residual emulsion or aberration in the film medium are eliminated and other defects such as scratches and dirt have to exceed an adjustable noise threshold before they are detected. Second, the high frequency response is set by the scan rate and beam size and not limited by the characteristics of a light slit and a photo linear detector.
Different methods of scanning are possible:
1. Scanning beam of light and a fixed photo detector
a. Cathode ray tube and a photo detector.
b. Mirrored galvanometer and a photo detector.
c. Scanning LED array and a photo detector.
2. Fixed slit of light and a scanning photo detector
a. Fixed slit of light and a Videcon camera tube.
b. Fixed spot of light and a charge coupled device (CCD).
The above methods are possible alternatives but the preferred method is (b) of group 2. It consists of a light source, not a slit, and a CCD linear scanner. In this configuration a spot of light just slightly larger than the width of the sound track is directed toward the sound track of one side of the film and the reverse side of the illuminated area is focused with a lens on the active area of the CCD linear scanner. The effective slit width of the scanner is reduced by the magnification value of the lens. For example, the CCD scanner has an aperture of 13 um and the lens has a magnification value of 2.25 which effectively reduces the slit to 6 um giving a maximum frequency response of 20.0 KHZ, more than double the present value. The scan frequency of the CCD device is set by the electronics used to drive it and is chosen to produce the desired output band width.
In preferred embodiment, the system includes a small halogen lamp with integrated condensing lens or light emitting diode or laser diode as a light source to produce a uniform light spot. The lens on the other side of the film picks up the back lighted image of the sound track and magnifies it 2.25 times then projects the resulting larger image of the sound track onto the image sensor in the scanning photo detector (CCD). The scanning photo detector effectively scans across the track image and converts the image of the sound track into one or two pulses depending on whether the film is mono or stereo. As the film moves past the photo sensor the sound tracks vary in width and the output pulses from the scanning photo detector vary in width accordingly. The scanning photo detector electronically scans the magnified image 40,000 times per second which sets the rate of the output pulse or pulses. The scanning photo sensor consists of an integrated row of photo sensors in a silicon chip numbering 512 which are sequentially read by the on chip scan circuitry. As each of the 512 photo sensors is read an output voltage is produced from each which is proportional to the amount of light falling on it and when all are combined a video signal is produced which is a profile of the light intensity across the scanned image. The effective width of the scan line made by the photo sensor array is 6 um or 0.00024 inches. This line thickness determines the resolution or frequency response of the system. The very best present systems have a maximum frequency response of about 10 KHZ, and are followed by a high frequency boost amplifier to achieve a 16 to 18 KHZ response. The novel analog sound track digitizer has a response of 20 KHZ requiring no boost circuits.
The scanning photo detector output is connected to a pulse width modulation (PWM) electronics circuit where the pulse stream is converted into one or two channels of audio. It is then transmitted to pre amps and amplifiers of the existing theater sound system. The sound thus produced is noise-free, has an increased frequency response, has an expanded dynamic range and produces an increased clarity of dialogue.
Normally there are only two sound tracks on 35mm film for stereo sound. Some older films have only one sound track. The number of pulses from the scanner for each scan is equal to the number of sound tracks on the particular film being used. For stereo sound two pulses are generated. Each of the two pulses from the scanner which are varying in width relative to the sound tracks are steered to a separate circuit where it is integrated with respect to time and converted to an electrical audio signal.
The pulses from the scanner have only two discrete levels zero and maximum. The two levels represent no light or full light levels on the scanner active area. This provides a noise threshold wherein the noise sources on the film must have a contrast equal to the sound track area contrast or be ignored by the scanner. By its nature this eliminates emulsion residue and plastic film medium at the aberration effects on the output signal. Other sources of noise which cause unwanted pulse width modulation are required to exceed another threshold set by the integrating circuitry before conversion to audio. A potentiometer in the circuitry of the PWM electronics box can be adjusted to vary the threshold level from zero to 100%. By using 20% of the amplitude, any noise has to be 80% of the total amplitude before it will be sensed at all.
The improved analog sound track digitizer has incorporated a novel weave tracking circuit to follow any side to side movement of the sound track due to film processing errors or projector problems. Another circuit has been added which allows the scanner to track minimum width tracks that are as little as {fraction (1/4+L )} the minimum standard set by the industry. An automatic gain control (AGC) has been added to the video amplifier circuit which allows for changes of 10 to 1 in light intensity and or film density. Also incorporated therein is a new circuit for comparing the video signal with a reference signal. The new video comparator has been designed to allow the user to tune the (cross-modulation) distortion inherent on a film print to zero. Additionally, new noise canceling circuits are included which work independently to remove the effect of breaks or scratches as large as {fraction (1/10+L )} of an inch on the film and blotches of any size.